Why cleaning mould off a reveal never works for long, and what actually changes the conditions.
Black mould in the corners of a window reveal is a symptom, not a fault in itself. It grows where a cold surface meets moist air for long enough.
That means cleaning it treats the evidence and not the cause. This guide covers the three conditions behind it and what changes each.
Mould needs all three, so removing any one of them helps:
In the order we'd usually recommend:
Single glazing, failed sealed units and old metal frames all create surfaces cold enough to condense on almost daily in winter. If mould reappears within weeks of cleaning and the frames are old, the glazing is likely part of the cause.
If cold glazing is part of the problem, we'll show you what changes and what won't.
They remove one of the three ingredients by warming the surface, which usually makes a large difference. Ventilation still matters — new windows are much better sealed than old ones.
They cause a small, controlled amount of air exchange. Closing them entirely is one of the most common causes of condensation problems in otherwise good windows.
Persistent mould in a home is a health concern and worth addressing properly rather than repeatedly wiping away. If it's widespread, get the property assessed.